Artist's concept of NASA's TRMM satellite operating in Earth orbit. Photo: NASA

New research shows that adding rainfall data from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite and other meteorological satellites to forecast models can more than triple the accuracy of short-term rainfall forecasts.

These findings by researchers at Florida State University, Tallahassee, FL, were presented Wednesday at the annual American Meteorological Society's (AMS) meeting in Long Beach, CA, and will be featured in an upcoming edition of the Journal of Climate.

In addition, researchers found that using the rainfall data collected from defense meteorological satellites and TRMM can be used to increase the forecast accuracy even further. Their method examines the behavior of a number of different forecast models and selects those properties from each model that lead to the true rainfall as observed by the TRMM satellite. These model properties are then used to predict the rainfall for three days into the future, with remarkable success.

"Including rainfall into the multi-forecast model, or 'superensemble' model, is a unique approach," said Prof. T.N. Krishnamurti, the paper's lead author and a TRMM scientist at Florida State University. "Overall we're most interested in improving the rainfall three-day forecast accuracy. Our research has shown that the accuracy of global and regional forecasts using the superensemble is higher with TRMM research data."

These forecast results are based on five experiments each conducted Aug. 1 to Aug. 5, 1998. The forecast accuracy was higher over all tropical regions. Scientists attribute this success to a combination of improved analyses available from the superensemble approach as well as the availability of accurate rainfall estimates over the tropics from the TRMM satellite.

Map of global rainfall as measured by TRMM. Photo: NASA

For years, scientists have attempted to improve the short- term forecasts in the tropics, but only minor improvements were made. Now, with the research data from the NASA spacecraft, scientists will more accurately forecast rainfall in the region. This is particularly important when it comes to hurricane tracks and rainfall accumulations. Experimental forecasts made by this new technique during the 1999 hurricane season, for example, correctly forecast the track of major hurricanes such as Dennis and Floyd.

Scientists have a keen interest in how potential changes in the global climate might affect the associated rainfall patterns as they in turn affect human activities. "Making such improvements in even the short-term forecasts is important because it demonstrates that we are learning more about the behavior of rainfall within these models," said Chris Kummerow, the spacecraft project scientist at Goddard Space Flight Center, Greenbelt, MD. "Understanding rainfall patterns generated by our global climate models is an extremely difficult problem. Having additional information available from these weather forecast models has the obvious benefit of better short term forecasts, and may help shed additional light upon the climate models."

TRMM is NASA's first mission dedicated to observing and understanding tropical rainfall and how it affects the global climate. The TRMM spacecraft fills an enormous void in the ability to calculate worldwide precipitation because ground-based radars that measure precipitation cover so little of the planet. Ground-based radars cover only 2 percent of the area covered by TRMM, said Kummerow.

TRMM has produced continuous data since December 1997. Tropical rainfall, which falls between 35 degrees north latitude and 35 degrees south latitude, comprises more than two-thirds of the rainfall on Earth. Previous estimates of tropical precipitation were usually made on the basis of weather models and occasional inclusion of very sparse surface rain gauges and/or relatively few measurements from satellite sensors. The TRMM satellite allows these measurements to be made in a focused manner.

TRMM, a NASA-Japanese mission, is part of NASA's Earth Science Enterprise, a long-term research program designed to study the Earth's land, oceans, air, ice and life as a total system.